1. Field of the Invention
This invention relates to multi-layered microwave-absorptive materials for use, for example, in ships, aircrafts and other mobile structures.
2. Prior Art
Microwave-absorptive materials are broadly classified into an attenuation type in which the energy of incident wave attenuates as it passes through the absorber, and a matching type in which the amount of reflection of incident wave on the front surface of the absorber and that of reflection of wave from a reflector on the rear surface of the absorber are controlled so as to in effect eliminate the generation of reflected wave.
A typical matching-type wave absorber well known in the art comprises an absorbing layer made up of a synthetic resin or rubber sheet carrying magnetic ferrite (Fe.sub.2 O.sub.3). This type of material excels of course in wave absorption, but is rather heavy, hence unsuitable for light-weight applications and furthermore structurally weak as it is a resinous or rubber sheet simply admixed with ferrite. Attempts have been made to produce a light-weight microwave absorber from a rubber composition having carbon black particles dispersed therein in place of ferrite. Such wave absorbers, however, have been effective only in handling a narrow band of microwave frequencies. It has been known that wave absorption over wide frequency bands can be achieved by the use of multi-layered absorbers, a typical example of which comprises a low impedance layer usually free of ferrite or carbon black, a high impedance layer containing substantial amounts of ferrite or carbonblack, and a reflection layer containing the same metal or fibrous carbon, these layers being laminated or otherwise bonded together in the order mentioned so that the impedance of each layer increases progressively in the direction of incidence of a microwave. Such a multilayered structure has a drawback in that, when the layers each particularly composed of a heat-resistant matrix resin, are laminated and cured simultaneously, each layer undergoes strain upon cooling to room temperature due to the pressure of considerable internal stresses resulting from the differences in the thermal coefficient of contraction between the different layers.